The present invention relates to formulations for delivery of parasiticides. More particularly, disclosed are encapsulated parasiticides, a method of their production, and methods of treatment using these encapsulated parasiticides. The parasiticides are encapsulated within nonphospholipid lipid vesicles which are themselves dispersed or suspended in an oil or water-based medium.
The parasiticide formulations of the present invention are useful to prevent or eradicate infestation by a broad spectrum of pests, e.g., insects, acarines, and helminths. Most existing formulations of parasiticides use organic solvents in order to solubilize the parasiticides because they are generally insoluble in water. Using organic solvents for this purpose is problematic at best. The solvents are irritating to the skin, thereby requiring the user to wear bulky protective clothing. Organic solvents add significantly to the cost of the parasiticides so solubilized, and the parasiticides must then be stored in metal containers to prevent container breakdown. Further, despite these harsh conditions, many potentially advantageous parasiticides cannot be solubilized in high enough concentrations to render them suitable for use, and ordinarily cannot be used in combination with other parasiticides, which would increase efficacy and spectrum coverage, due to chemical incompatibility.
The formulations of the present invention can be either water or oil-based. The advantages of the present formulation over those using organic solvents are that the new formulations are less expensive, less hazardous, and less irritating. Storage is simpler in that the formulation may be packed in plastic rather than the previously obligatory metal containers. Further, using the formulations of the present invention will allow otherwise incompatible combinations of active ingredients, either in separate vesicles or having one encapsulated and one in solution, to be used.
Lipid vesicles have not been considered particularly good carriers for water-insoluble materials such as parasiticides because of the instability of the lipid vesicles when carrying large quantities of lipophilic material. In addition, industrial use of lipid vesicles have been limited because of cost factors which are too high. Lipid vesicles are substantially spherical structures made of materials having a high lipid content, e.g., surfactants or phospholipids, organized in the form of lipid bilayers. The lipid bilayers encapsulate an aqueous volume which is either interspersed between multiple onion-like shells of lipid bilayers (forming multilamellar lipid vesicles or "MLV") and/or the aqueous volume is contained within an amorphous central cavity. The most commonly known lipid vesicles having an amorphous central cavity filled with aqueous medium are the unilamellar lipid vesicles. Large unilamellar vesicles ("LUV") generally have a diameter greater than about 1 .mu.while small unilamellar lipid vesicles ("SUV") generally have a diameter of less than 0.2 .mu.. There are a variety of uses for lipid vesicles including the use as adjuvants or as carriers for a wide variety of materials.
Although substantially all the investigation of lipid vesicles in recent years has centered on multilamellar and the two types of unilamellar lipid vesicles, a fourth type of lipid vesicle, the paucilamellar lipid vesicle ("PLV"), exists. This lipid vesicle has barely been studied heretofore and until recently, has only been manufactured previously with phospholipids. PLV's consist of about 2 to 10 peripheral bilayers surrounding a large, unstructured central cavity. Normally, this central cavity was filled with an aqueous solution. See Callo and Mc Grath, Cryobiology 1985, 22(3), pp. 251-267.
Each type of lipid vesicle appears to have certain uses for which it is best adapted. For example, MLV's have a higher lipid content than any of the other lipid vesicles so to the extent that a lipid vesicle can carry a lipophilic material in the bilayers without degradation, MLV's have been deemed more advantageous than LUV's or SUV's for carrying lipophilic materials. In contrast, the amount of water encapsulated in the aqueous shells between the lipid bilayers of the MLV's is much smaller than the water which can be encapsulated in the central cavity of LUV's, so LUV's have been considered advantageous in transport of aqueous material. However, LUV's, because of their single lipid bilayer structure, are not as physically durable as MLV's and are more subject to enzymatic degradation. SUV's have neither the lipid or aqueous volumes of the MLV's or LUV's but because of their small size have easiest access to cells in tissues.
PLV's, which can be considered a sub-class of the MLV's, are a hybrid having features of both MLV's and LUV's. PLV's appear to have advantages as transport vehicles for many uses as compared with the other types of lipid vesicles. In particular, because of the large unstructured central cavity, PLV's are easily adaptable for transport of large quantities of aqueous-based materials. Also as illustrated in previously cited U.S. patent application Ser. No. 157,571, filed Mar. 3, 1988 and now U.S Pat. No. 4,911,928, the aqueous cavity of the PLV's can be filled wholly or in part with an apolar oil or wax which acts as a vehicle for the transport or storage of hydrophobic materials. The amount of hydrophobic material which can be transported by the PLV's with an apolar core is much greater than can be transported by MLV's. The multiple lipid bilayers of the PLV's provides PLV's with additional capacity to transport lipophilic material in their bilayers as well as with additional physical strength and resistance to degradation as compared with the single lipid bilayer of the LUV's.
All of the early lipid vesicle or liposome studies used phospholipids as the lipid source for the bilayers. The reason for this choice was that phospholipids are the principal structural components of natural membranes. However, there are many problems using phospholipids as artificial membranes. Isolated phospholipids are subject to degradation by a large variety of enzymes. The most easily available phospholipids are those from natural sources, e.g., egg yolk lecithin, which contain polyunsaturated acyl chains that are subject to autocatalytic peroxidation. When peroxidation occurs, the lipid structure breaks down, causing premature release of encapsulated materials and the formation of toxic peroxidation byproducts. This problem can be avoided by hydrogenation but hydrogenation is an expensive process, thereby raising the cost of the starting materials. A kilogram of egg yolk lecithin pure enough for pharmacological liposome production presently costs in excess of $1,000. This is much to high a cost for a starting material for most applications. Even less highly purified phospholipids are too expensive for most animal uses.
Recently, there has been some indication, particularly from L'Oreal and Micro Vesicular Systems, Inc., that commercially available surfactants might be used to form the lipid bilayer in liposome-like multilamellar lipid vesicles. Both surfactants and phospholipids are amphiphiles, having at least One lipophilic acyl or alkyl group attached to a hydrophilic head group. The head groups are attached to one or more lipophilic chains by ester or ether linkages. Commercially available surfactants include the Brij family of polyoxyethylene acyl ethers, the SPAN sorbitan alkyl esters, and the TWEEN polyoxyethylene sorbitan fatty acid esters, all available from ICI Americas, Inc. of Wilmington, Delaware.
The methods and materials disclosed herein for producing the paucilamellar lipid vesicles all yield vesicles with a high aqueous or oil volume. Electron micrographs confirm that the paucilamellar lipid vesicles are distinct from the LUV's and the classic MLV's
Accordingly, an object of the invention is to provide oil or water-based formulations for carrying parasiticides.
Another object of the invention is to provide formulations having parasiticides encapsulated within nonphospholipid vesicles
A further object of the invention is to provide a method of preparing formulations of substantially water-insoluble parasiticides which exhibit antiparasitic action.
A still further object of the invention is to provide a method of treatment of plants, animals or their products to provide antiparasitic action.
These and other objects and features of the invention will be apparent from the following description.